1. Field of the Invention
This invention relates to a method and apparatus for creating a clean working area while protecting workers and the environment from being contaminated by hazardous products used or created therein, and in particular to the protection of a wet chemical bench.
2. Description of Related Art
Operations are often performed in clean hoods in industries such as electronics or pharmaceuticals where air is filtered within a chamber and passed over a work area. This is done under positive pressure with respect to the pressure outside the chamber, or else dirt particles would be sucked into the work area when it is necessary to enter or remove articles or when an operator needs to work within the chamber. These hoods are cheaper to install and operate than in maintaining a whole laboratory in "Class 1" condition, where the class rating represents the number of particles greater than 0.5 micrometers per cubic foot. In normal laboratories this number is at least 100,000.
U.S. Pat. No. 5,259,812 (D. A. Kleinsek) is directed to a laboratory room and an anteroom within a chamber, separated by a dividing wall having a door. Both rooms form a positive pressure entry system which prevents outside air from entering the clean room. A containment center within a clean room is also described which employs conventional iris ports shielded with two layers rubber and a double door transition box to remove samples without allowing external air from entering into the containment center.
U.S. Pat. No. 5,255,710 (D. Palmer) teaches two stage control of air flow to an environment from a pressure source wherein a piston defines an aperture through which air may flow from the pressure source and a plenum so as to variably constrict the conduit, the weight of the piston tending to move it in a direction to lessen the piston's impedance to air flow. An adjustable valve, located between the piston and the environment, further impedes the flow of air. A gate may be rigidly attached to the piston so that changes in the pressure source's strength do not affect the plenum pressure.
U.S. Pat. No. 5,029,518 (F. X. Austin) describes modular wall sections which are assembled on site to construct a wall of a clean room. Each section serves as an air return and directs air from the room upward within the section to a negative pressure plenum within the ceiling of the clean room.
U.S. Pat. No. 4,880,581 (F. R. Dastoli et al.) describes the placement of a shroud over a portion of a device, and U.S. Pat. No. 4,682,927 (P. R. Southworth et al.) discloses the use of a conveyer system to move cassettes of semiconductor wafers between clean rooms.
With increased regulatory requirements for the protection of workers and the environment there remains a need to protect the work from dust while providing access to the work area and completely removing toxic chemicals or pathogens used or generated within the work area. A means is needed to deal with the apparently contradicting requirements of positive pressure in the work area to prevent dust ingress but negative pressure to prevent escape of toxic substances. Such equipment should also not require continuous monitoring of flow conditions.
3. Summary of the Invention
The present invention relates to apparatus and a method to provide access for parts or an operator's hands into a clean area while ensuring that toxic contaminants within the work area do not escape into the room. This is accomplished by creating an isolation region of pressure immediately behind the entrance to the chamber that is higher than the pressure outside the chamber. This keeps dirt from entering, but alone, it would also expel toxic materials. Therefore, another region of pressure is maintained within the chamber, a working pressure, which is less than the isolation pressure near the entrance to the chamber. This pressure differential prevents toxic contaminants from escaping.
In one embodiment of the invention, an uncontaminated gas is supplied to one extremity of an enclosure at a supply pressure, and an isolation pressure is maintained inside the enclosure near an aperture to the enclosure to create an isolation region. A working pressure is maintained behind the isolation region to create a working region. The enclosure is evacuated by maintaining another extremity at an exhaust pressure. The aforementioned pressures are maintained in descending order to create a pressure gradient and flow to remove contaminants. The isolation pressure is also maintained at a pressure higher than that outside the chamber. A fraction of the air in the isolation region escapes through the aperture to keep particles out, and a fraction of it also flows into the working region to keep contaminants within the chamber. Placing a baffle before the isolation region and the working region creates the desired pressure differential. The baffle may provide no impedance to flow over the isolation region, or the impedance may vary across the depth of the baffle.
In another embodiment of the invention, a vane intercepts a portion of the air flowing within the chamber to create the required pressure inequality to keep dirt out of the chamber and to keep toxic contaminants within it. The vane may be fixed in position or can be made to vary in position to optimize the flow for each installation.
The advantage to creating the isolation region of pressure which is higher than the outside pressure and the working pressure in most of the chamber is that toxic contaminants within the chamber are prevented from escaping through the aperture and that dirt particles from the outside are prevented from destroying the work, while access to the chamber is provided for the entry and removal of articles or for human intervention. Once the pressure differentials are established by the use of baffles or vanes, no further calibration is needed except to maintain sufficient flow to keep the isolation pressure above the outside pressure.
These and other features and advantages of the invention will be better understood with consideration of the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.